Sometimes it is necessary to enforce some PCB parts with some structural support.
The most common method of providing that support is to use a flex PCB stiffener and coverlay. When it comes to flex circuits and rigid-flex PCBs, there are a few different options out there when it comes to stiffeners.
In general, a stiffener is a thin, rectangular piece of non-conductive material. In rigid-flex circuits, it is also called a coverlay, which connects the conductive traces (or vias) on both sides of the board. It helps prevent the formation of cracks and voids in the traces due to bending. We can place the stiffener on both sides or only one side of the rigid-flex PCB.
Flexible PCB Stiffener Requirements
It is important to note that stiffener must be compliant with all design rules of the PCB manufacturing process. Many PCB manufacturers do not make custom flex PCB stiffeners for customers. So, if you must use a specific or design for your PCBs, you should consult with the manufacturing company. It helps ensure that the manufacturer will provide a custom stiffener as needed. Requirements and limitations of flex PCB stiffener can vary from one manufacturer to another. It depends on their control system and requirements, such as OSHA and UL restrictions.
- It must consist of non-conductive material such as PTFE, PET, and sheet metal.
- The thickness should be less than 0.5 mm and less than or equal to 2 mm.
- It should not cause permanent deformation in the rigid PCB when applied under a specified load of flex circuits and one side only.
- The stiffener must not be sharp.
- It should not cause a failure in the rigid PCB when flex circuits and you apply only one side under a specified load of flex circuits and one side.
- The stiffness should be between 0.25-0.75 MPa (15-40 psi).
- It should be compatible with the other layers, printed circuit boards, and components used in rigid-flex circuits and rigid PCBs.
- The stiffener should have a smooth surface finish to reduce damage to the overall flexible circuit board fabrication process during manufacture.
Things to Know About Flex PCB Stiffeners
Stiffeners do not have to consist of a single material. Some companies, such as RayMing PCB and Assembly, can help you find and purchase a material that meets all your requirements for stiffeners and coverlays. Here are a few things to keep in mind about flex PCB stiffeners:
- A flex PCB stiffener does not have to be continuous. However, we can use it if it is continuous and does not cause any problems with the design or creating the rigid-flex circuit.
- Flex PCB stiffener must consist of a material that provides enough stiffness to support the weight of the rigid-flex circuits. Still, it should not interfere with their operation or cause any negative effects.
- You do not need to attach stiffener does not need to the rigid PCB. However, we can attach it with glue, bolts, nuts, or screws.
- We can only use A flex PCB stiffener if compatible with the rigid-flex circuit design and manufacturing process.
- The amount of flex PCB stiffener needed depends on the final features of the rigid-flex PCB.
- It must comply with any design rules the manufacturing company specifies and not interfere with its processes.
- A preferred method for attaching a flex PCB stiffener to a rigid-flexible circuit without adhesives is to use bolts and nuts or screws.
- It must comply with any design rules the manufacturing company specifies and not interfere with its processes.
Types of Stiffeners
We have three main types of stiffeners: Stainless Steel Stiffener, FR4 Stiffener, and PI Stiffener. We commonly use them in different locations on the PCB. If we design a PCB for rigid-flex circuits, it must comply with the manufacturing process and rules. If the manufacturer did not design the PCB for rigid-flex circuits, it would be an excellent option to use PI Stiffener.
Polyimide Stiffener (PI)
We use PI stiffeners in almost all rigid-flex circuits and other stiffeners such as FR4, PET, and PTFE stiffeners. These stiffeners can be easily cut to size with a pair of scissors or knives without wasting any material. These are not the best for PCBs requiring changing the lead pitch or other design rules.
PI stiffeners consist of a PI (Polyimide) layer laminated between two PET (Polyester) layers. The thickness varies from a few hundred microns to about 1mm. Manufacturers make PI stiffeners as one single piece, and they do not recommend that you separate them with any other material. We can cut Polyimide stiffeners to size only with a pair of scissors or a knife. We cannot snap them. PI stiffeners are non-conductive and brittle.
Molded PI Stiffener
The molded PI stiffener comprises two layers, PET and PI, formed into one tight piece for rigidity. But the manufacturing method is different from that of the pi stiffener. It consists of two layers laminated into one piece by double-sided tape. The benefit of using molded PI stiffener is that we can easily adjust it to the desired size. Then you can cut them from a larger piece of PI Stiffener. The disadvantage of the molded PI stiffener is that they are more expensive. If the price is the main concern here, it would be better to use pi stiffeners.
Molded PI stiffeners consist of two PET and PI layers formed into one piece. We mold the PI layer with the PET layer. The thickness of the molded PI Stiffener flex PCB is much thicker than a pi stiffener. It is economical and has several benefits. We can cut these to size easily without wasting material.
Manufacturers do not recommend PI Stiffeners when your lead pitch or you need to change other design rules. You cannot adjust them like flexible PCB stiffeners or FR4 flex stiffeners. We can use them only after establishing the rigid-flex circuit design.
PI stiffeners are not conductive, non-adhesive, and brittle. We use them for different rigid-flex circuits. Some examples include cellular phones, microwave ovens, battery-powered units, and military electronics. They must be lightweight and carry enough force to prevent PCB deformation.
Stainless Steel Stiffener
The stainless-steel stiffener has a unique design for surface mounting applications. We often use them for rigid digital circuits applications. We can produce it by sintering stainless steel powder into a billet. To make the material stronger, we add more steel to it. Silicone is the preferred adhesive material because it gives good adhesion to stainless steel. We commonly use these stiffeners in ruggedized systems, military electronics, and medical devices. They withstand tough environmental conditions such as rough handling, vibration, and temperature extremes.
Stainless steel stiffeners are great for rigid-flex circuits where the circuit is very sensitive to external factors, thermal expansion, and mechanical shock. They are very durable and yet light in weight.
Stainless steel stiffeners are mainly helpful in rigid digital circuits where physical damage to the PCB is an important design criterion. We can easily apply them during either the assembly process or on the surface through the coating of silicones. But these are expensive. So we should only consider them if you need them.
Stainless steel stiffeners are essential in military electronics and medical devices in ruggedized systems. They are perfect for flexible circuit manufacturing, although we do not use them in rigid-flex circuits.
FR4 Stiffener
The FR4 stiffener is one of the best stiffeners for rigid-flex circuits. This is because it has a wide range of applications, and we can apply them to many rigid-flex circuits. Flexible PCB designers use FR4 stiffener for rigid electronic circuits. Some examples are cellular phones, microwave ovens, and battery-powered units. This material has a flexible quality that we can change by mechanical bending without chemical additives or treatments. We commonly use it for rigid digital circuits and other rigid-flex circuits, which is a key requirement.
FR4 Stiffener consists of a material called FR4. Manufacturers make the FR4 from a combination of stainless steel, glass, and resin. The main advantages include high tensile strength, low permeability, good mechanical strength, low permeability, and good adhesion. FR4 flex Stiffener is also a lightweight material that helps in reducing the weight of the rigid-flex circuit.
FR4 stiffeners are good for rigid-flex circuits when you need a large amount of stiffness, low static dissipation, and low environmental stress cracking. They are also preferred for high-frequency applications because they do not affect impedance matching characteristics.
FR4 stiffeners are affordable, easy to handle, and easy to apply on flexible circuits. They are also helpful for rigid-flex circuits such as cellular phones, microwave ovens, and battery-powered units.
Flexible PCB stiffeners are the most used stiffeners for flexible circuit manufacturing. The main advantage of these stiffeners is that you can use them for rigid-flex circuit assembly without any problem. They are easy to handle, and we can easily apply them either by hot air lamination or solvents in solvent-based assembly.
Stainless Steel/Aluminum Stiffener flex PCB
Manufacturers make the stainless steel/aluminum stiffener by laminating stainless steel foil to a flexible PCB material. We mainly use this for military electronics, cellular phones, and microwave ovens. They can withstand tough environmental conditions. These are helpful when there are no rigid-flex circuit design limitations.
The main advantage of using this stiffener is that we can easily shape it according to the need, size, and shape of the PCB and other components. Stainless steel and aluminum are helpful as an additive to increase the strength of the PCB without adding more material. This is easy to handle, and we can apply them in a large quantity if needed. We can shape it using the most commonly available materials such as water, heat, or solvent.
Most people prefer Stainless steel/aluminum stiffeners for rigid-flex circuits. It is useful when you need a large amount of stiffness, low static dissipation, and low environmental stress cracking.
Multiple PCB Stiffener
The multiple PCB stiffener provides the PCB design with desired rigidity and flexibility. We can use Multiple PCB stiffeners for different rigid-flex circuits. It requires different stiffness types.
The main advantage of using this stiffener is that you do not need to make any changes in the design of the rigid-flex circuit. This is especially when it comes to multiple PCB stiffeners. Each rigid-flex circuit has unique stiffness properties that we need to consider while designing a flexible circuit.
Multiple PCB stiffeners are relatively expensive, so we should use them only if you need them. Also, there may be other limitations in using multiple PCB stiffeners due to their weight and design complexity.
We can use the multiple PCB stiffener for rigid-flex circuits where you need a large amount of stiffness and low environmental stress cracking.
PCB Stiffener Thickness
Stiffeners help increase the PCB’s rigidity and provide it with desired stiffness property. When choosing a stiffener for your rigid-flex circuit, you must decide whether you want a flexible PCB or a rigid-flex circuit. So, first, you must decide what type of stiffener you need.
For flexible circuits, use a flexible PCB stiffener. Examples include FR4 Stiffener, Flexible Glass Filled Stiffeners, and Flexible Foam Stiffeners. These are the most used for flexible circuits.
Use a rigid PCB stiffener for rigid-flex circuits such as Aluminum Stiffener and Stainless-Steel Stiffeners. These are the most used for rigid-flex circuits.
You also must decide how thick you want your stiffener to be. This is because a thicker PCB stiffener can add stiffness and increase the PCB’s material strength, while a thinner one might not be as effective.
The most used stiffener thicknesses for flexible circuits are 1.5 mm to 2.5 mm.
For rigid-flex circuits, you must decide what kind of stiffness. You should also determine the rigidity you need for each specific rigid-flex circuit you want to design/build. You can use any stiffness or rigidity level per your and application requirements.
The toughest topic is to decide on the correct PCB stiffener thickness. If you are designing a pcbway flex circuit and using multiple PCB stiffeners, you must choose the correct overall stiffness for the rigid-flex circuit. We call this a trade-off between the weight of the rigid-flex circuit, stiffness, and cost.
Stiffener Trade-Off
Number of Stiffeners vs. PCB Thickness
For any rigid-flex circuit design, you must choose a balance between the number of stiffeners you want to use for your rigid-flex circuit design and the thickness of each PCB. The thicker the PCB stiffener is, the more weight you add to your PCB.
For example, if you have a 1.5 mm rigid-flex circuit and use multiple PCB stiffeners, then the total thickness of all the PCB stiffeners should not be more than 2 mm.
If you have a thick rigid-flex circuit and multiple PCB stiffeners, the total thickness of all the stiffeners would be more than 2 mm. The weight of each stiffener changes as per its thickness.
Coverlay flex PCB Materials & Construction
Coverlay is a covering material that one can apply over an electronic board to protect it from corrosion or fluids. A coverlay can be either liquid or solid.
The most common materials used for the construction of a coverlay are:
We use polyimide, silicones, epoxies, and polyesters in large quantities as they are easy to use and have good properties. But you need to consider the toxicity and other environmental factors while using some of these materials.
The rigidity of a flex coverlay depends on the thickness and flexibility of the material. The different types of coverlays are essential in different applications. They depend on the user’s needs, environmental conditions, etc.
After assembling and testing the PCB, it needs to undergo “Burn-in.” This ensures that you remove any manufacturing defects found in running tests after some time. Burn-in is very important in electronics because all production defects will show up. Another thing that can happen during the burn-in process is the breakdown of existing solder joints. The most common things to happen during burn-in are:
After the burn-in process, we test and measure all electronic components. We do this process using different testing equipment. The is a lot we can do with the assembled PCB after burn-in. The most common things include:
At this stage, the first thing that happens is that the PCB and we visually check all its parts to make sure they are perfect for shipping or storage. We then perform all the final electrical tests for the electronic components to ensure no production defects are present.
FPC Coverlay in Flex Circuits
FPC Coverlay is a flexible circuit near the solder mask side that improves the flexible circuit’s electrical, optical, and mechanical characteristics.
Typically, we make flex circuits by printing on various materials that have dielectric properties. With high temperatures and pressures, we make flex circuits in this process. Some of these layers tend to warp or curl up at the corners when they cool down during this process. This may affect the electrical and mechanical characteristics of your flex circuitry.
We use FPC coverlay to minimize the effects of this warping or curling up of the layers.
FPC Coverlay allows you to design a flexible circuit with good mechanical characteristics. It also protects it from environmental effects.
Common Coverlay Configurations
With the rapid development of technological advancements, companies introduce many new flexible printed circuit patterns every day. The main reason behind this is that every new model requires a different set of flexible printed circuit patterns for its design and manufacture.
Therefore, you need to consult with a PCB manufacturer about your requirements before opting for any design. This is because there are chances of choosing the wrong technology. The rule of thumb for selecting a flexible printed circuit pattern is that its design should be compatible with the process. It should also be compatible with materials used for making a flexible circuit. You should also understand the various materials and structures used to make different components.
How to Choose the Correct Coverlay PCB
FPC coverlay depends on the materials used for making flex circuits.
Solder mask is a polyamide base material that can easily etch using chemicals. Its flexibility makes it a good option for making FPC coverlay. We can use Kapton Tape as an FPC coverlay because it has high resistance to heat and high strength properties.
FPC Coverlays allow you to design a flexible circuit board. It will have improved mechanical, electrical, and optical characteristics.
We can use Flex circuits in various industries. For example, consumer electronics, aerospace, defense, automotive and medical, and others. From children’s toys to pacemakers, all use the same FPC concept.
FPC Assembly and Testing
Flex PCB assembly and test are essential in manufacturing a flexible circuit. For example, if you are designing a product prototype, then the flexibility of that product would be essential. Therefore, you would need to design a product prototype that is flexible and has good assembly requirements. If your prototype is not flexible enough, it could break or tear easily during its use. So, to make your prototype more durable and flexible, you should use rigid-flex circuit technology.
FPC assembly and test requirements are very different from rigid circuit assembly and test. Flex circuits have many extra components such as FPC, vias, and pads. Therefore, you must select an FPC material that can withstand heat and pressure during the soldering process.
It is advisable not to use rapid thermal or high bending cycles during the assembly process as per the IPC-610 Class-2 standard. This is because it would affect the thickness and quality of your flexible circuits. Flex circuits must go through some flexing and folding cycles in this process.
Standard FPC assembly requirements
Flex circuit assembly involves cutting, stretching, folding, and bending your flexible circuit components. During this process, you need to apply high temperatures and bending forces to your flexible components. This process can damage your flexible printed circuit boards if not done properly. However, we recommend using several thin layers to apply force evenly across the entire board.
To avoid polyamide dielectric material from cracking or breaking during this process, you should use a heat-resistant material. We can also use several heat-resistant materials. For example, polyamide, polyimide, and silicon nitride make flex circuits.
To ensure that the flexible circuit coverlay has minimal impact on the overall assembly process and protects your components from environmental effects, you should use FPC coverlay in your design.
Most customers prefer to have real-time images of their Flex PCB assemblies on their computer monitors. This helps them to understand the process and avoid mistakes.
Therefore, you should adequately illuminate your assembly environment with less or no dust. You should also use low-temperature plastic fixtures to assemble your flex circuit boards.
The correct way of flex circuit testing involves using a burn-in system for detecting any weak links in the components and other assembly faults.
This system depends on the IPC-610 Class 3 standard. It involves testing all individual components of flexible printed circuit boards before assembling them.
Conclusion
This article provides a fundamental understanding of Flex PCB design and how you handle it by using different stiffeners to make the circuit flexible. Flex PCB is a new product that has manufacturers have been developing over recent years. We use it in many product applications with physical limitations on flex circuits, such as size, form factor, and weight. Flex PCB design has its unique requirements, which are essential to consider while designing a flexible circuit on a rigid PCB.
